1. Field of the Invention
The present invention relates to a vehicle alternator or an electric power generator to be mounted on a vehicle such as a passenger car and a truck.
2. Description of the Related Art
In general, a vehicle alternator is driven by an engine such as an internal combustion engine mounted on a vehicle through a driving belt that joins the vehicle alternator and the engine. Such a vehicle alternator is driven by the rotary force provided from the engine through the driving belt in order to generate an electric power.
A speed increasing ratio of the vehicle alternator is determined based on a ratio between a groove diameter of a crankshaft pulley and a groove diameter of a pulley of the vehicle alternator.
The vehicle alternator is usually driven at the speed increasing ratio of approximately two or three times the number of revolutions of the vehicle engine. In general, the number of revolutions of the vehicle engine is within a range of approximately 700 rpm (revolutions per minute) to 7,000 rpm (as the maximum value). Because the speed increasing ratio of the vehicle alternator is approximately two or three times of that of the vehicle engine, the number of revolutions of the vehicle alternator takes a wide range of approximately 14,000 rpm to 21,000 rpm (as the maximum value).
However, the distribution of the number of revolutions of a common vehicle engine has approximately 700 rpm in idling condition which is 40% of the whole and also has approximately 1,300 rpm in a low speed revolution which is 50% of the whole during actual running of the vehicle such as 10-15 mode that is a typical running condition in view of the calculation of fuel consumption. That is, because the low speed of revolutions of the vehicle engine from the number of revolutions in idling condition to 1,300 rpm takes approximately 90% of the whole, the actual vehicle running takes the number of revolutions of not more than 1,300 rpm.
Accordingly, this means that it is necessary to keep a large amount of electric power in a vehicle alternator when the vehicle alternator is using a low speed of revolution.
FIG. 6 is a view showing a typical output characteristic of a widely used vehicle alternator. FIG. 6 indicates that the vehicle alternator has a low electrical power generation capacity during a low speed of revolution. In order to increase the magnitude of electric power generation during the low speed of revolution, in other words, in order to enhance the electric power generation capability during the low speed of revolution, it is necessary to either increase the speed increasing ratio by using a simple speed-increasing mechanism obtained by fixing the internal gear, or increase a volume of the vehicle alternator. Japanese patent laid open publication JP H6-105513 has disclosed such a conventional technique to increase the speed increasing ratio of a vehicle alternator. (see pages 2 to 3, and FIG. 1 to FIG. 2 in JP H6-105513)
However, although a high capability of electric power generation of the vehicle alternator is achieved on increasing the speed increasing ratio thereof as the former case, this manner decreases the lifetime of wear parts such as brushes and bearings which form the vehicle alternator, and further serves to break rotating components such as cooling fans because the vehicle alternator rotates at a high speed. In addition, because iron loss to heat the components of the vehicle alternator is generally in proportion to the square of a power generation frequency (as the number of revolutions), the heating value of the vehicle alternator at a higher rotating speed is greater than that at a lower rotating speed even if in generation of the same amount of electric power.
Thus, the vehicle alternator at a higher rotating speed causes various troubles such as thermal stress and wear stress, and those troubles decrease the reliability of the vehicle alternator. If a vehicle alternator consists of various components of high reliability and durability in order to avoid those troubles and drawbacks, the manufacturing cost of the vehicle alternator is increased because the cost of each assembling components with high reliability and durability is generally high.
On the other hand, the latter case to increase the volume of the vehicle alternator brings to decrease or deteriorate the mounting flexibility of a vehicle and to introduce the difficulty of designing the engine to be assembled to the vehicle, and to break a stay on which various components are mounted because the total weight of the vehicle alternator is increased. This decreases the reliability of the vehicle. In order to eliminate the deterioration of the vehicle reliability, each part and component are reinforced, the manufacturing cost is also increased, similar to the case of increasing the durability and thermal resistance of each component. As a result, this trial brings to increase the manufacturing cost of the vehicle alternator.